Composite high reflectivity layer

ABSTRACT

A high efficiency light emitting diode with a composite high reflectivity layer integral to said LED to improve emission efficiency. One embodiment of a light emitting diode (LED) chip comprises an LED and a composite high reflectivity layer integral to the LED to reflect light emitted from the active region. The composite layer comprises a first layer, and alternating plurality of second and third layers on the first layer, and a reflective layer on the topmost of said plurality of second and third layers. The second and third layers have a different index of refraction, and the first layer is at least three times thicker than the thickest of the second and third layers. For composite layers internal to the LED chip, conductive vias can be included through the composite layer to allow an electrical signal to pass through the composite layer to the LED.

This application is a continuation of, and claims the benefit of, U.S.patent application Ser. No. 12/316,097 to Ibbetson et al., filed on Dec.8, 2008 now U.S. Pat. No. 7,915,629, and having the same title as thepresent application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to light emitting diodes, and to light emittingdiodes with a high reflectivity contact and method for forming thecontact.

2. Description of the Related Art

Light emitting diodes (LED or LEDs) are solid state devices that convertelectric energy to light, and generally comprise one or more activelayers of semiconductor material sandwiched between oppositely dopedn-type and p-type layers. When a bias is applied across the dopedlayers, holes and electrons are injected into the active layer wherethey recombine to generate light. Light is emitted from the active layerand from all surfaces of the LED.

For typical LEDs it is desirable to operate at the highest lightemission efficiency, and one way that emission efficiency can bemeasured is by the emission intensity in relation to the input power, orlumens per watt. One way to maximize emission efficiency is bymaximizing extraction of light emitted by the active region of LEDs. Forconventional LEDs with a single out-coupling surface, the externalquantum efficiency can be limited by total internal reflection (TIR) oflight from the LED's emission region. TIR can be caused by the largedifference in the refractive index between the LED's semiconductor andsurrounding ambient. Some LEDs have relatively low light extractionefficiencies because the high index of refraction of the substratecompared to the index of refraction for the surrounding material, suchas epoxy. This difference results in a small escape cone from whichlight rays from the active area can transmit from the substrate into theepoxy and ultimately escape from the LED package. Light that does notescape can be absorbed in the semiconductor material or at surfaces thatreflect the light.

Different approaches have been developed to reduce TIR and improveoverall light extraction, with one of the more popular being surfacetexturing. Surface texturing increases the light escape probability byproviding a varying surface that allows photons multiple opportunitiesto find an escape cone. Light that does not find an escape conecontinues to experience TIR, and reflects off the textured surface atdifferent angles until it finds an escape cone. The benefits of surfacetexturing have been discussed in several articles. [See Windisch et al.,Impact of Texture-Enhanced Transmission on High-Efficiency SurfaceTextured Light Emitting Diodes, Appl. Phys. Lett., Vol. 79, No. 15,October 2001, Pgs. 2316-2317; Schnitzer et al. 30% External QuantumEfficiency From Surface Textured, Thin Film Light Emitting Diodes, Appl.Phys. Lett., Vol 64, No. 16, October 1993, Pgs. 2174-2176; Windisch etal. Light Extraction Mechanisms in High-Efficiency Surface TexturedLight Emitting Diodes, IEEE Journal on Selected Topics in QuantumElectronics, Vol. 8, No. 2, March/April 2002, Pgs. 248-255; Streubel etal. High Brightness AlGaNInP Light Emitting Diodes, IEEE Journal onSelected Topics in Quantum Electronics, Vol. 8, No. March/April 2002].

U.S. Pat. No. 6,657,236, also assigned to Cree Inc., disclosesstructures formed on the semiconductor layers for enhancing lightextraction in LEDs.

Another way to increase light extraction efficiency is to providereflective surfaces that reflect light so that it contributes to usefulemission from the LED chip or LED package. In a typical LED package 10illustrated in FIG. 1, a single LED chip 12 is mounted on a reflectivecup 13 by means of a solder bond or conductive epoxy. One or more wirebonds 11 connect the ohmic contacts of the LED chip 12 to leads 15Aand/or 15B, which may be attached to or integral with the reflective cup13. The reflective cup may be filled with an encapsulant material 16which may contain a wavelength conversion material such as a phosphor.Light emitted by the LED at a first wavelength may be absorbed by thephosphor, which may responsively emit light at a second wavelength. Theentire assembly is then encapsulated in a clear protective resin 14,which may be molded in the shape of a lens to collimate the lightemitted from the LED chip 12. While the reflective cup 13 may directlight in an upward direction, optical losses may occur when the light isreflected. Some light may be absorbed by the reflector cup due to theless than 100% reflectivity of practical reflector surfaces. Some metalscan have less than 95% reflectivity in the wavelength range of interest.

FIG. 2 shows another LED package in which one or more LED chips 22 canbe mounted onto a carrier such as a printed circuit board (PCB) carrier,substrate or submount 23. A metal reflector 24 mounted on the submountsurrounds the LED chip(s) 22 and reflects light emitted by the LED chips22 away from the package 20. The reflector 24 also provides mechanicalprotection to the LED chips 22. One or more wirebond connections 11 aremade between ohmic contacts on the LED chips 22 and electrical traces25A, 25B on the submount 23. The mounted LED chips 22 are then coveredwith an encapsulant 26, which may provide environmental and mechanicalprotection to the chips while also acting as a lens. The metal reflector24 is typically attached to the carrier by means of a solder or epoxybond. The metal reflector 24 may also experience optical losses when thelight is reflected because it also has less than 100% reflectivity.

The reflectors shown in FIGS. 1 and 2 are arranged to reflect light thatescapes from the LED. LEDs have also been developed having internalreflective surfaces to reflect light internal to the LEDs. FIG. 3 showsa schematic of an LED chip 30 with an LED 32 mounted on a submount 34 bya metal bond layer 36. The LED further comprises a p-contact/reflector38 between the LED 32 and the metal bond 36, with the reflector 38typically comprising a metal such as silver (Ag). This arrangement isutilized in commercially available LEDs such as those from Cree® Inc.,available under the EZBright™ family of LEDs. The reflector 38 canreflect light emitted from the LED chip toward the submount back towardthe LED's primary emitting surface. The reflector also reflects TIRlight back toward the LED's primary emitting surface. Like the metalreflectors above, reflector 38 reflects less than 100% of light and insome cases less than 95%. The reflectivity of a metal film on asemiconductor layer may be calculated from the materials' opticalconstants using thin film design software such as TFCalc™ from SoftwareSpectra, Inc. (www.sspectra.com).

FIG. 4 shows a graph 40 showing the reflectivity of Ag on galliumnitride (GaN) at different viewing angles for light with a wavelength of460 nm. The refractive index of GaN is 2.47, while the complexrefractive index for silver is taken from the technical literature. [SeeHandbook of Optical Constants of Solids, edited by E. Palik.] The graphshows the p-polarization reflectivity 42, s-polarization reflectivity44, and average reflectivity 46, with the average reflectivity 46generally illustrating the overall reflectivity of the metal for thepurpose of LEDs where light is generated with random polarization. Thereflectivity at 0 degrees is lower than the reflectivity at 90 degrees,and this difference can result in up to 5% or more of the light beinglost on each reflection. In a LED chip, in some instances TIR light canreflect off the mirror several times before it escapes and, as a result,small changes in the mirror absorption can lead to significant changesin the brightness of the LED. The cumulative effect of the mirrorabsorption on each reflection can reduce the light intensity such thatless than 75% of light from the LED's active region actually escapes asLED light.

SUMMARY OF THE INVENTION

The present invention discloses a higher reflectivity layer for use inLEDs and LED chips to increase emission efficiency. One embodiment of alight emitting diode (LED) chip according to the present inventioncomprises an active region between two oppositely doped layers. Acomposite high reflectivity layer is arranged to reflect light emittedfrom the active region. The composite layer comprises a first layer, oneor more second layers, and a plurality of third layers on the firstlayer. The second layers have an index of refraction different from thethird layers. The second and third layers alternate, and each of thethird layers having different thicknesses compared to the other of thethird layers. A reflective layer is included on the topmost of thesecond and third layers

Another embodiment of an LED chip according to the present inventioncomprises a submount with an LED mounted to the submount. A compositehigh reflectivity layer is arranged between the submount and the LED toreflect LED light. The composite layer comprises a plurality of layersand a conductive path through the composite layer through which anelectrical signal can pass to the LED.

Another embodiment of an LED chip according to the present inventioncomprises an LED and a composite high reflectivity layer integral to theLED to reflect light emitted from the active region. The composite layercomprises a first layer, and alternating plurality of second and thirdlayers on the first layer. The second and third layers have a differentindex of refraction, and the first layer is at least three times thickerthan the thickest of the second and third layers.

In still another embodiment of an LED chip according to the presentinvention comprises an LED and a composite high reflectivity layerintegral to the LED to reflect light emitted from the LED. The compositelayer comprising a first layer, and one or more second layers and aplurality of third layers on the first layer, with the second and thirdlayers comprising incomplete second and third layer pairs.

These and other aspects and advantages of the invention will becomeapparent from the following detailed description and the accompanyingdrawings, which illustrate by way of example the features of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of one embodiment of a prior art LED lamp;

FIG. 2 is a sectional view of another embodiment of a prior art LEDlamp;

FIG. 3 is a sectional view of another embodiment of a prior art LEDchip;

FIG. 4 is a graph showing the reflectivity of a metal reflector atdifferent viewing angles;

FIG. 5 a is a sectional view of one embodiment of an LED chip at afabrication step in one method according to the present invention;

FIG. 5 b is a sectional view of the LED chip in FIG. 5 a at a subsequentfabrication step;

FIG. 6 is a sectional view of one embodiment of composite layeraccording to the present invention;

FIG. 7 is a graph showing the reflectivity of a composite layeraccording to the present invention;

FIG. 8 is a graph showing the reflectivity of composite layer accordingto the present invention;

FIG. 9 is a sectional view of another embodiment of a composite layeraccording to the present invention;

FIG. 10 a is a sectional view of another embodiment of an LED accordingto the present invention;

FIG. 10 b is a sectional view of the LED in FIG. 10 a at a subsequentfabrication step;

FIG. 10 c is a sectional view of the LED in FIG. 10 b at a subsequentfabrication step;

FIG. 10 d is a sectional view of the LED in FIG. 10 c at a subsequentfabrication step;

FIG. 11 is a plan view of one embodiment of a composite layer accordingto the present invention;

FIG. 12 is a plan view of another embodiment of a composite layeraccording to the present invention;

FIG. 13 a is a sectional view of another embodiment of an LED chipaccording to the present invention;

FIG. 13 b is a sectional view of the LED chip shown in FIG. 13 a at asubsequent fabrication step;

FIG. 14 is a sectional view of another embodiment of an LED chipaccording to the present invention; and

FIG. 15 is a sectional view of still another embodiment of an LED chipaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to solid-state emitters and methodsfor fabricating solid-state emitters having one or more composite highreflectivity contacts or layers arranged to increase emission efficiencyof the emitters. The present invention is described herein withreference to light emitting diodes (LED or LEDs) but it is understoodthat it is equally applicable to other solid-state emitters. The presentinvention can be used as a reflector in conjunction with one or morecontacts, or can be used as a reflector separate from the contacts.

The improved reflectivity of the composite contact/layer (“compositelayer”) reduces optical losses that can occur in reflecting light thatis emitted from the active region in a direction away from useful lightemission, such as toward the substrate or submount, and also to reducelosses that can occur when TIR light is reflecting within the LED.Embodiments of the present invention provide various unique combinationsof layers that can comprise a composite layer. In one embodimentaccording to the present invention, the composite layer can comprise afirst relatively thick layer, with second and third layers havingdifferent indices of refraction and different thickness, and areflective layer. The composite layer can be in many different locationssuch as on an outer surface of the LED or internal to the LED.

Different embodiments of the invention also provide composite layershaving conductive via or path arrangements that provide conductive pathsthrough the composite layer. This allows an electric signal to passthrough the composite layer along the vias so that the composite layercan be used as an internal layer, where an electrical signal passesthrough the composite layer during operation. This via arrangement cantake many different shapes and sizes as described in detail below.

The present invention is described herein with reference to certainembodiments but it is understood that the invention can be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. In particular, the composite layer cancomprise many different layers of different material with many differentthicknesses beyond those described herein. The composite layer can be inmany different locations on different solid-state emitters beyond thosedescribed herein. Further, the composite layer can be provided with orwithout conductive structures to allow electrical signals to passthrough.

It is also understood that when an element such as a layer, region orsubstrate is referred to as being “on” another element, it can bedirectly on the other element or intervening elements may also bepresent. Furthermore, relative terms such as “inner”, “outer”, “upper”,“above”, “lower”, “beneath”, and “below”, and similar terms, may be usedherein to describe a relationship of one layer or another region. It isunderstood that these terms are intended to encompass differentorientations of the device in addition to the orientation depicted inthe figures.

Although the terms first, second, etc. may be used herein to describevarious elements, components, regions, layers and/or sections, theseelements, components, regions, layers and/or sections should not belimited by these terms. These terms are only used to distinguish oneelement, component, region, layer or section from another region, layeror section. Thus, a first element, component, region, layer or sectiondiscussed below could be termed a second element, component, region,layer or section without departing from the teachings of the presentinvention.

Embodiments of the invention are described herein with reference tocross-sectional view illustrations that are schematic illustrations ofembodiments of the invention. As such, the actual thickness of thelayers can be different, and variations from the shapes of theillustrations as a result, for example, of manufacturing techniquesand/or tolerances are expected. Embodiments of the invention should notbe construed as limited to the particular shapes of the regionsillustrated herein but are to include deviations in shapes that result,for example, from manufacturing. A region illustrated or described assquare or rectangular will typically have rounded or curved features dueto normal manufacturing tolerances. Thus, the regions illustrated in thefigures are schematic in nature and their shapes are not intended toillustrate the precise shape of a region of a device and are notintended to limit the scope of the invention.

FIGS. 5 a and 5 b show one embodiment of an LED chip 50 according to thepresent invention, and although the present invention is described withreference to fabrication of a single LED chip it is understood that thepresent invention can also be applied to wafer level LED fabrication,fabrication of groups of LEDs, or fabrication of packaged LED chips. Thewafer or groups of LEDs can then be separated into individual LED chipsusing known singulation or dicing methods. This embodiment is alsodescribed with reference to an LED chip having vertical geometryarrangement and that is flip chip mounted. As further described belowthe present invention can be used with other LED arrangements, such aslateral geometry LEDs and non flip-chip orientations.

The LED chip 50 comprises an LED 52 that can have many differentsemiconductor layers arranged in different ways. The fabrication andoperation of LEDs is generally known in the art and only brieflydiscussed herein. The layers of the LED 52 can be fabricated using knownprocesses with a suitable process being fabrication using MOCVD. Thelayers of the LED 52 generally comprise an active layer/region 54sandwiched between n-type and p-type oppositely doped epitaxial layers56, 58, all of which are formed successively on a growth substrate 60.It is understood that additional layers and elements can also beincluded in the LED 52, including but not limited to buffer, nucleation,contact and current spreading layers as well as light extraction layersand elements. The active region 54 can comprise single quantum well(SQW), multiple quantum well (MQW), double heterostructure or superlattice structures.

The active region 54 and layers 56, 58 can be fabricated from differentmaterial systems, with preferred material systems being Group-IIInitride based material systems. Group-III nitrides refer to thosesemiconductor compounds formed between nitrogen and the elements in theGroup III of the periodic table, usually aluminum (Al), gallium (Ga),and indium (In). The term also refers to ternary and quaternarycompounds such as aluminum gallium nitride (AlGaN) and aluminum indiumgallium nitride (AlInGaN). In one embodiment, the n- and p-type layers56, 58 are gallium nitride (GaN) and the active region 54 comprisesInGaN. In alternative embodiments the n- and p-type layers 56, 58 may beAlGaN, aluminum gallium arsenide (AlGaAs) or aluminum gallium indiumarsenide phosphide (AlGaInAsP) and related compounds.

The growth substrate 60 can be made of many materials such as sapphire,silicon carbide, aluminum nitride (AlN), GaN, with a suitable substratebeing a 4H polytype of silicon carbide, although other silicon carbidepolytypes can also be used including 3C, 6H and 15R polytypes. Siliconcarbide has certain advantages, such as a closer crystal lattice matchto Group III-nitrides than sapphire and results in Group III-nitridefilms of higher quality. Silicon carbide also has a very high thermalconductivity so that the total output power of Group-III nitride deviceson silicon carbide is not limited by the thermal dissipation of thesubstrate (as may be the case with some devices formed on sapphire). SiCsubstrates are available from Cree Research, Inc., of Durham, N.C. andmethods for producing them are set forth in the scientific literature aswell as in U.S. Pat. Nos. Re. 34,861; 4,946,547; and 5,200,022.

Different embodiments of the LED 52 can emit different wavelengths oflight depending on the composition of the active region 54 and n- andp-type layer 56, 58. In the embodiment shown, the LED 50 emits a bluelight in the wavelength range of approximately 450 to 460 nm. The LEDchip 50 can also be covered with one or more conversion materials, suchas phosphors, such that at least some of the light from the LED passesthrough the one or more phosphors and is converted to one or moredifferent wavelengths of light. In one embodiment, the LED chip emits awhite light combination of light from the LED's active region and lightfrom the one or more phosphors.

In the case of Group-III nitride devices, current typically does notspread effectively through the p-type layer 58 and it is known that athin current spreading layer 64 can cover some or the entire p-typelayer 58. The current spreading layer helps spread current from thep-type contact across the surface of the p-type layer 58 to provideimproved current spreading across the p-type layer with a correspondingimprovement in current injection from the p-type layer into the activeregion. The current spreading layer 64 is typically a metal such asplatinum (Pt) or a transparent conductive oxide such as indium tin oxide(ITO), although other materials can also be used. The current spreadinglayer can have many different thicknesses, with one embodiment of an ITOspreading layer a thickness of approximately 115 nm. The currentspreading layer 64 as well as the layers that comprise the compositelayer described below can be deposited using known methods. It isunderstood that in embodiments where current spreading is not a concern,the composite layer can be provided without a current spreading layer.

Referring now to FIG. 5 b, a composite high reflectivity layer 62 can bedeposited on the p-type layer 58, and in the embodiment shown thecurrent spreading layer is between the reflectivity layer 62 and thep-type layer. The composite layer 62 according to the present inventionhas higher reflectivity to the wavelength of light generated by the LED52 compared to standard metal contacts or distributed Bragg reflectors(DBRs). The composite layer generally comprises a thick layer ofmaterial followed by a plurality of thinner layers that combine toprovide improved reflectivity. The present invention provides acomposite layer with the desired reflectivity that also minimizes thenumber of layers to minimize the manufacturing complexities and cost.

Referring now to FIG. 6, the different layers that can comprise oneembodiment of a composite layer according to the present invention areshown, but it is understood that many different materials, thicknessesand number of layers can also be used. A first layer 66 of the compositelayer is provided on the current spreading layer 64, and the first layercan comprise many different materials, with the preferred materialcomprising a dielectric. Different dielectric materials can be used suchas a SiN, SiO₂, Si, Ge, MgOx, MgNx, ZnO, SiNx, SiOx, alloys orcombinations thereof, with the material in first layer 66 in theembodiment shown comprising SiO₂. This first layer 66 should berelatively thick to provide a reliable viewing angle cut-off point afterwhich the reflectivity of the composite layer is approximately 100%, andin one embodiment used with a blue emitting LED the first layer 66 canhave a thickness in the range of 500 to 650 nm, with one embodimenthaving a thickness of approximately 591 nm.

Referring now to the graph 72 in FIG. 7, which shows the p-polarizationreflectivity 74, s-polarization reflectivity 76, and averagereflectivity 78 at different viewing angles for a first layer 66 havinga thickness in the range of 500 to 650 nm for blue wavelengths of light.The viewing angle cut-off is at a viewing angle of approximately 36degrees. That is, the reflectivity of the composite layer 62 isapproximately 100% at viewing angles greater than approximately 36degrees, and below this viewing angle the reflectivity can be as low as94% at certain viewing angles.

Referring again to FIG. 6, to improve reflectivity at lower viewingangles and to improve the angle averaged reflectivity (AAR), thecomposite layer 62 can also comprise second layers 68 a, 68 b and thirdlayers 70 a, 70 b with the second and third layers being made ofmaterials with differing indexes of refraction. In different embodimentsdifferent materials can be used for the layers and a different number oflayers can be included, with the embodiment shown having two secondlayers 68 a-b comprising SiO₂ and two third layers 70 a-b comprisingTiO₂. SiO₂ has an index of refraction of 1.46, while TiO₂ has an indexof refraction of 2.34. The two SiO₂ layers can have differentthicknesses and the two TiO₂ layers can have different thicknesses,which provide a composite layer that is different from standard DBRswhere the layers of different materials have the same thickness. Onesuch example of this type of DBR is a ¼ wavelength DBR where each of thesecond and third SiO₂ and TiO₂ layers can have essentially the sameoptical thickness approximately equal to a ¼ wavelength of the light. Inother embodiments of the composite layer, Ta₂O₅ can be used in place ofTiO₂.

For the composite layer embodiment shown that is used in conjunctionwith a blue emitting LED, the second layers 68 a-b can have thicknessesin the range of 100 to 120 nm, and approximately 40 to 60 nmrespectively, with one embodiment of the second layers beingapproximately 108 nm and 53 nm thick. The third TiO₂ layers 70 a-b canhave thicknesses in the range of 55 to 75 nm and 35 to 55 nm,respectively, with one embodiment having thicknesses of approximately 65nm and 46 nm respectively.

The composite layer 62 can also comprise a reflective layer 71 on thesecond layer 68 b, deposited using known methods such as sputtering. Thereflective layer 71 can have many different thicknesses and can comprisemany different reflective materials, with suitable materials being Ag,Al and Au. The choice of material can depend on many factors with onebeing the wavelength of light being reflected. In the embodiment shownreflecting blue wavelengths of light, the reflective layer can compriseAg having a thickness of approximately 200 nm. In other embodiments thereflective layer 71 can comprise composite metal layers such as TiAg,NiAg, CuAg or PtAg, and in some embodiments these composite layers canprovide improved adhesion to the layer it is formed on, such as thesecond layer 68 b. Alternatively, a thin layer of material such asindium tin oxide (ITO), Ni, Ti or Pt can be included between the secondlayer 68 b and the reflective layer to also improve adhesion.

The structure of the composite layer 62 provides improved AAR comparedto standard ¼ wavelength DBRs. Although there may be a number of reasonswhy this arrangement provides this improvement, it is believed that onereason is that the different thicknesses of the second layers 68 a,68 band the third layers 70 a, 70 b present differently to light at variousincident angles. That is, light will reach composite layer 62 at manydifferent angles, and at these different angles the second layers 68 a,68 b and third layers 70 a, 70 b can appear as different thicknesses,such as multiples of a ¼ wavelength thickness depending on the angle. Itis believed that the different thicknesses provide the best overall AARacross viewing angles of 0-90 degrees.

FIG. 8 is a graph 80 showing the reflectivity of a composite layersimilar to that shown in FIG. 6, and shows the p-polarizationreflectivity 82, s-polarization reflectivity 84, and averagereflectivity 86 at different viewing angles. In this case, thereflectivity includes the effect of an ITO current spreading layer 64having a thickness of 115 nm and finite absorption coefficient of500/cm, which results in the reflectivity approaching but being slightlybelow 100% at viewing angles greater than approximately 36 degrees. TheAAR across viewing angles of 0-90 degrees for the composite layer shownis approximately 98.79%, which provides an improvement over a standardDBR with a similar number of layers made of the same materials. Atcertain wavelengths of light the AAR of a standard ¼ wavelength DBR canbe approximately 98.73% or less. This difference can have a significantimpact on overall LED brightness because light can reflect off thecomposite layer multiple times before escaping from the LED. Thiscompounding effect of multiple reflections amplifies even smalldifferences in reflectivity.

FIG. 9 shows another embodiment of the composite layer 100 that issimilar to composite layer 62 described above and can be used with ablue emitting LED. The composite layer 100 can have four layers insteadof five. In this embodiment the first layer 102 is on a currentspreading layer 64, although it is understood that the composite layer100 can be used without a current spreading layer. The first layer 102is similar to the first layer 66 described above and can be made of manymaterials and many different thicknesses. In the embodiment shown thefirst layer 102 can comprise SiO₂ with thickness in the range of 500 to650 nm, with one embodiment having a thickness of approximately 591 nm.

In this embodiment, the composite layer 100 comprises only one secondlayer 106 sandwiched between two third layers 108 a-b. Like theembodiment above. That is, there are not an equal number of alternatingsecond layers and third layers as in composite layer 62 described above,and as in conventional DBRs. This results in second and third layerscombinations that comprise incomplete pairs or that are asymmetric. Inembodiments with incomplete second and third layer pairs can comprisedifferent numbers of each layer such as two second layers and threethird layers, three second layers and four third layers, etc.

The second and third layers 106, 108 a-b can comprise many differentmaterials and can have many different thicknesses. In the embodimentshown, the second layer 106 can comprise SiO₂ and can have a thicknessin the range of approximately 100 to 120 nm, with one embodiment havinga thickness of 107 nm. The third layers 108 a-b can comprise TiO₂ andcan have thicknesses of in the range of 45 to 65 nm and 65 to 85 nmrespectively, with one embodiment having third layer thicknesses ofapproximately 56 and 75 nm, respectively. The composite layer 100 canalso comprise a reflective layer 110 on the third layer 108 b that canbe deposited using known methods and can comprise the same materials asreflective layer 71 described above.

By having an asymmetric arrangement, the composite layer can have fewerlayers with the corresponding reduction in manufacturing steps andcosts. This can also provide the additional advantage of better adhesionto subsequent layers, such as a reflective layer 110. In this embodimentthe top layer comprises third layer 108 b, which is TiO₂. This materialcan provide improved adhesion to reflective metals compared to thesecond layer 106 comprising SiO₂. The composite layer 100, however, canhave a reduced AAR compared to a six-layer arrangement shown in FIG. 6,with the AAR of one embodiment of a five-layer arrangement as shown inFIG. 9 being approximately 98.61%. This, however, represents animprovement over a standard five layer DBR where the AAR can beapproximately 96.61%. Similar to the six-layer embodiment above, thisdifference can have a significant impact on overall LED brightnessbecause of the compounding effect of multiple reflections.

It is understood that composite layers according to the presentinvention can have many different layers of different materials andthicknesses. In some embodiments the composite layer can comprise layersmade of conductive materials such as conductive oxides. The conductiveoxide layers can have different indices of refraction and the differingthicknesses to provide the improved reflectivity. The differentembodiments can have different arrangements of complete and incompletepairs of second and third layers. It is also understood that thecomposite layer can be arranged in different locations on an LED and cancomprise different features to provide thermal or electrical conductionthrough the composite layer.

Referring now the FIGS. 10 a through 10 d, another embodiment of an LED120 having many of the same features as LED 50 shown in FIGS. 5 a and 5b and for those same features the same reference numbers will be used.The LED 50 is fabricated such that is can be arranged in a flip-chiporientation, so for this embodiment the end LED chip will have thecomposite layer 62 (or composite layer 100) arranged as an internallayer as further described below. Accordingly, an electric signal shouldpass through the composite layer 62. Referring now to FIG. 10 a and FIG.11, holes 122 can be formed through the composite layer 62 at random orregular intervals, with the holes sized and positioned so that aconductive material can be deposited in the holes to form conductivevias. In the embodiment shown the holes 122 are at regular intervals.

In different embodiments having a current spreading layer 64, the holes122 may or may not pass through the current spreading layer 64. Theholes 122 can be formed using many known processes such as conventionaletching processes or mechanical processes such as microdrilling. Theholes 122 can have many different shapes and sizes, with the holes 122in the embodiment shown having a circular cross-section with a diameterof approximately 20 microns. Adjacent holes 122 can be approximately 100microns apart. It is understood that the holes 122 (and resulting vias)can have cross-section with different shapes such as square,rectangular, oval, hexagon, pentagon, etc. In other embodiments theholes are not uniform size and shapes and there can be different spacesbetween adjacent holes.

Referring now to FIG. 12, instead of holes an interconnected grid 124can be formed through the composite layer 62, with a conductive materialthen being deposited in the grid 124 to form the conductive path throughthe composite layer. The grid 124 can take many different forms beyondthat shown in FIG. 11, with portions of the grid interconnecting atdifferent angles in different embodiment. An electrical signal appliedto the grid 124 can spread throughout along the interconnected portions.It is further understood that in different embodiments a grid can beused in combination with holes.

Referring now to FIG. 10 b, a conductive layer 126 can be deposited onthe composite layer 62 covering its reflective layer and filling theholes 122 to form vias 128 through the composite layer 62. In otherembodiments, the conductive layer can cover less than all of thecomposite layer 62. The conductive layer 126 can comprise many differentmaterials such as metals or conductive oxides, both of which can bedeposited using known techniques.

Referring now to FIG. 10 c, the LED 120 can be flip-chip mounted to asubmount 130 using known mounting techniques. In the embodiment shown,the LED 50 is flip-chip mounted to the submount by a conductive bondmaterial 132. It is understood that in embodiments where the LEDs chips120 are formed at the wafer level and then singulated, the LEDs chips120 can be wafer bonded to the submount 130 using known wafer bondingtechniques. The submount 130 can be made of many different materials andcan have many different thicknesses, with the preferred submount 130being electrically conductive so that an electrical signal can beapplied to the active region of the LED through the submount 130. Thesignal also passes through the composite layer along conductive vias128.

Referring now to FIG. 10 d, the growth substrate 60 (as shows in FIG. 10c) can be removed using known grinding and/or etching processes. A firstcontact 134 can be deposited on the n-type layer 56 and a second contact136 can be deposited on the submount 130. The first and second contacts134, 136 can comprise many different materials such as Au, copper (Cu)nickel (Ni), indium (In), aluminum (Al), silver (Ag), or combinationsthereof. In still other embodiments the first and second contacts cancomprise conducting oxides and transparent conducting oxides such asITO, nickel oxide, zinc oxide, cadmium tin oxide, indium oxide, tinoxide, magnesium oxide, ZnGa₂O₄, ZnO₂/Sb, Ga₂O₃/Sn, AgInO₂/Sn, In₂O₃/Zn,CuAlO₂, LaCuOS, CuGaO₂ and SrCu₂O₂. The choice of material used candepend on the location of the contacts as well as the desired electricalcharacteristics such as transparency, junction resistivity and sheetresistance. The top surface of the n-type layer 56 can be textured orshaped such as by laser texturing, mechanical shaping, etching (chemicalor plasma), scratching or other processes, to enhance light extraction.

During operation, an electrical signal is applied to the LED 50 acrossfirst and second contacts 134, 136. The signal on the first contact 134spreads into the n-type layer 56 and to the active region 54. The signalon the second contact 136 spreads into the submount 130, throughcomposite layer 62 along the vias 128, through the current spreadinglayer 64, into the p-type layer 58 and to the active region 54. Thiscauses the active region 54 to emit light and the composite layer 62 isarranged to reflect light emitted from the active region toward thesubmount 128, or reflected by TIR toward the submount 130, back towardthe top of the LED chip 50. The composite layer 62 encourages emissiontoward the top of the LED chip 50 and because of its improvedreflectivity, reduces losses that occur during reflection.

It is understood that the composite layers can be used in many differentways and in many different locations on LEDs, LED chips, and othersolid-state emitters. As shown in FIGS. 13 a and 13 b, the compositelayers can be used in conjunction with a lateral geometry LED chip 150where both contacts are on one side of the LEDs. The layers of the LED150 are generally the same as those for LED chip 50 and can comprise anactive layer/region 154 sandwiched between n-type and p-type oppositelydoped epitaxial layers 156, 158, all of which are formed successively ona growth substrate 160. For lateral geometry LEDs, a portion of thep-type layer 158 and active region 154 is removed, such as by etching,to expose a contact mesa 161 on the n-type layer 156. In thisembodiment, a composite layer 162 similar to the composite layer 62described above can be included on both the surface of the n-type layer156 and the surface of the p-type layer 158, with the composite layerhaving a metal layer 164 and conductive vias 166 similar to the metallayer 126 and vias 128 described above.

Referring now to FIG. 13 b, the LED chip 150 can be flip-chip mounted toa submount 168 using known mounting processes preferably by conductivebonds 170 to the metal layers 164 on the composite layers 162. Anelectrical signal from the submount 168 is applied to the LED throughthe conductive bonds 170, and composite layers 162, causing the LED chipto emit light. The composite layers 162 reflect light that is directedtoward the submount 168 to reflect back toward the emission surface ofthe LED chip 150. The improved reflectivity of the composite layers 162reduces reflectivity losses and improves overall emission efficiency ofthe LED chip 150.

FIG. 14 shows still another embodiment of LED 180 according to thepresent invention also having an active layer/region 184 sandwichedbetween n-type and p-type oppositely doped epitaxial layers 186, 188,all of which are formed successively on a growth substrate 190. Aportion of the p-type layer 188 and active region 184 is removed, suchas by etching, to expose a contact mesa on the n-type layer 186. In thisembodiment, p- and n-type contacts 192, 194 are deposited on the p-typelayer 188 and the contact mesa of the n-type 186, and a composite layer196 can be included on the bottom surface of the substrate 190.

In this embodiment, an electrical signal is not applied to the LEDthrough the composite layer 196. Instead, the electrical signal isapplied through the p- and n-type contacts 192, 194 where it spreadslaterally to the active region 184. As a result, an electrical signaldoes not need to pass through the composite layer 196 and the compositelayer 196 does not need electrically conductive vias. Instead, anuninterrupted composite layer can be included across the substratebottom surface to reflect light emitted from the active region towardthe substrate and TIR light that reflects toward the substrate. It isunderstood that in different embodiments the composite layer can alsocover all or part of the side surfaces of the LED 180, and a compositelayer can be used in with the n- and p-type contacts 192, 194 to improvetheir reflectivity.

It is also understood that a composite layer can also be used on thebottom surface of submounts in flip-chip embodiments where the submountsare transparent. In these embodiments the desired reflectivity can beachieved without having internal composite layers 162 as shown in FIG.13 b.

FIG. 15 shows still another embodiment of an LED chip 210 having anactive layer/region 214; n-type and p-type oppositely doped epitaxiallayers 216, 218, all of which are formed successively on a growthsubstrate 220. LED 210 has vertical geometry with a p-type contact 224on the p-type layer 218. A thin semitransparent current spreading layer(not shown) can cover some or the entire p-type layer 218, which istypically a metal such as Pt or a transparent conductive oxide such asITO, although other materials can also be used. A composite layer 226 isincluded on the substrate 220 and, because the LED 210 has verticalgeometry, an electrical signal can be applied to the LED through thecomposite layer 226. The composite layer comprises conductive vias 228similar to those described above that allow an electrical signal to passthrough the composite layer 226. The composite layer 226 can alsocomprise a metal layer 230 with an n-type contact 232 on the metallayer. This embodiment is particularly applicable to LEDs havingelectrically conductive substrates and an electrical signal applied tothe p-type 224 and n-type contact 232 spreads to the LED active region214 causing it to emit light. It is also understood that a compositelayer can be included with the p-type contact to improve itsreflectivity.

In different embodiments of the present invention the vias can serveadditional purposes beyond conducting electrical signals. In someembodiments the vias can be thermally conductive to assist in thermaldissipation of heat generated by the LED. Heat can pass away from theLED through the vias where it can dissipate.

Although the present invention has been described in detail withreference to certain preferred configurations thereof, other versionsare possible. Therefore, the spirit and scope of the invention shouldnot be limited to the versions described above.

We claim:
 1. A light emitting diode (LED) chip, comprising: an activeregion between two oppositely doped layers; a composite highreflectivity layer arranged to reflect light emitted from said activeregion, said composite layer comprising: a first layer; one or moresecond layers and a plurality of third layers on said first layer, saidsecond layers having an index of refraction different from said thirdlayers, wherein at least one of said second layers is interposed betweentwo of said third layers, and each of said third layers having differentthicknesses compared to the other of said third layers; and a reflectivelayer on the topmost of said second and third layers, wherein saidcomposite layer further comprises an interconnect grid.
 2. A lightemitting diode (LED) chip, comprising: an active region between twooppositely doped layers; a composite high reflectivity layer arranged toreflect light emitted from said active region, said composite layercomprising: a first layer; one or more second layers and a plurality ofthird layers on said first layer, said second layers having an index ofrefraction different from said third layers, wherein at least one ofsaid second layers is interposed between two of said third layers, andeach of said third layers having different thicknesses compared to theother of said third layers, wherein said first layer is thicker than thethickest of said second and third layers; and a reflective layer on thetopmost of said second and third layers.
 3. The LED chip of claim 2,wherein said first layer comprises a dielectric material.
 4. The LEDchip of claim 2, wherein said first layer is at least three timesthicker than the thickest of said second and third layers.
 5. The LEDchip of claim 2, wherein said first layer has a thickness in the rangeof 500 to 650 nanometers.
 6. The LED chip of claim 2, further comprisinga current spreading layer.
 7. The LED chip of claim 6, wherein saidcurrent spreading layer comprises indium tin oxide (ITO).
 8. The LEDchip of claim 2, wherein said one or more second layers comprise silicondioxide (SiO₂).
 9. The LED chip of claim 2, wherein said one or moresecond layers comprise two second layers.
 10. The LED chip of claim 9,wherein the first of said two second layers has a thickness in the rangeof 100 to 120 nanometers and the second of said two second layers has athickness in the range of 40 to 60 nanometers.
 11. The LED chip of claim2, wherein said plurality of third layers comprise titanium dioxide(TIO₂).
 12. The LED chip of claim 2, wherein said plurality of thirdlayers comprise two third layers.
 13. The LED chip of claim 12, whereinthe first of said two third layers has a thickness in the range of 55 to75 nanometers and the second of said two third layers has a thickness inthe range of 35 to 55 nanometers.
 14. The LED chip of claim 2, whereinsaid composite layer further comprises holes.
 15. The LED chip of claim14, wherein said holes are filled with a conductive material.
 16. Alight emitting diode (LED) chip, comprising: an active region betweentwo oppositely doped layers; a composite high reflectivity layerarranged to reflect light emitted from said active region, saidcomposite layer comprising: a first layer; one or more second layers anda plurality of third layers on said first layer, said second layershaving an index of refraction different from said third layers, whereinat least one of said second layers is interposed between two of saidthird layers, and each of said third layers having different thicknessescompared to the other of said third layers, wherein said first layer isthicker than the thickest of said second and third layers; and areflective layer on the topmost of said second and third layers, whereinsaid composite layer further comprises an interconnect grid.
 17. A lightemitting diode (LED) chip, comprising: an active region between twooppositely doped layers; a composite high reflectivity layer arranged toreflect light emitted from said active region, said composite layercomprising: a first layer; one or more second layers and a plurality ofthird layers on said first layer, said second layers having an index ofrefraction different from said third layers, wherein at least one ofsaid second layers is interposed between two of said third layers, andeach of said third layers having different thicknesses compared to theother of said third layers; a reflective layer on the topmost of saidsecond and third layers; and a current spreading layer between saidactive region and said composite layer.
 18. The LED chip of claim 17,wherein said first layer comprises a dielectric material.
 19. The LEDchip of claim 17, wherein said first layer is thicker than the thickestof said second and third layers.
 20. The LED chip of claim 19, whereinsaid first layer is at least three times thicker than the thickest ofsaid second and third layers.
 21. The LED chip of claim 17, wherein saidfirst layer has a thickness in the range of 500 to 650 nanometers. 22.The LED chip of claim 17, wherein said current spreading layer comprisesindium tin oxide (ITO).
 23. The LED chip of claim 17, wherein said oneor more second layers comprise silicon dioxide (SiO₂).
 24. The LED chipof claim 17, wherein said one or more second layers comprise two secondlayers.
 25. The LED chip of claim 24, wherein the first of said twosecond layers has a thickness in the range of 100 to 120 nanometers andthe second of said two second layers has a thickness in the range of 40to 60 nanometers.
 26. The LED chip of claim 17, wherein said pluralityof third layers comprise titanium dioxide (TIO₂).
 27. The LED chip ofclaim 17, wherein said plurality of third layers comprise two thirdlayers.
 28. The LED chip of claim 27, wherein the first of said twothird layers has a thickness in the range of 55 to 75 nanometers and thesecond of said two third layers has a thickness in the range of 35 to 55nanometers.
 29. The LED chip of claim 17, wherein said composite layerfurther comprises holes.
 30. The LED chip of claim 29, wherein saidholes are filled with a conductive material.
 31. A light emitting diode(LED) chip, comprising: an active region between two oppositely dopedlayers; a composite high reflectivity layer arranged to reflect lightemitted from said active region, said composite layer comprising: afirst layer; one or more second layers and a plurality of third layerson said first layer, said second layers having an index of refractiondifferent from said third layers, wherein at least one of said secondlayers is interposed between two of said third layers, and each of saidthird layers having different thicknesses compared to the other of saidthird layers; a reflective layer on the topmost of said second and thirdlayers; and a current spreading layer between said active region andsaid composite layer, wherein said composite layer further comprises aninterconnect grid.
 32. A light emitting diode (LED) chip, comprising: anactive region between two oppositely doped layers; a composite highreflectivity layer arranged to reflect light emitted from said activeregion, said composite layer comprising: a first layer; one or moresecond layers and a plurality of third layers on said first layer, saidsecond layers having an index of refraction different from said thirdlayers, wherein at least one of said second layers is interposed betweentwo of said third layers, each of said third layers having differentthicknesses compared to the other of said third layers, and wherein thenumber of said third layers is greater than the number of said secondlayers; and a reflective layer on the topmost of said second and thirdlayers.